The present invention relates to an image forming method and an image forming apparatus each for use in a copier, a facsimile, a printer, or the like. In particular, it relates to image formation performed by controlling a developer jumping from a developer holding member to a rear electrode by means of a printhead controlled by an image signal and causing the developer to adhere to an image receiving member positioned between the printhead and the rear electrode.
With recent improvements in the performance of personal computers and recent advances in networking technology, there have been growing demands for printers with high processing abilities which can handle not only voluminous documents but also color documents. However, an image forming apparatus capable of outputting monochrome and color documents of sufficiently high quality and having a high processing speed is still under development, the advent of which has been expected.
As one of technologies which implement such a high-performance image forming apparatus, there has conventionally been known an image forming technology which causes a toner to jump onto a recording sheet or image-forming means such as an intermediate image holding belt under the effect of an electric field.
As examples of an image forming apparatus of this type, there have been known image forming apparatus disclosed in Japanese Examined Patent Publication No. SHO 44-26333, U.S. Pat. No. 3,689,935 (Japanese Examined Patent Publication No. SHO 60-20747), Japanese Domestic-Phase PCT Patent Application No. HEI 9-500842, and the like.
Referring to FIG. 12, a description will be given to an image forming apparatus proposed in the specification and drawings of Japanese Patent Application No. HEI 10-100780 as an example of a similar image forming apparatus. In FIG. 12, 31 denotes a grounded toner holding member for holding and transporting a charged toner, 32 denotes a regulating blade for controlling the toner in one to three layers on the toner holding member 31, 33 denotes a supply roller for charging and supplying the toner to the toner holding member 31, and 34 denotes a printhead formed with a toner passage hole 35. A control electrode 36 is formed around the toner passage hole 36. A voltage corresponding to an image signal is applied from a control power supply 37, such as a driving IC, to the control electrode 36. 38 denotes a rear electrode, 39 denotes a power supply for the rear electrode 38, and 40 denotes receiving means such as a recording sheet transported on the rear electrode 38.
In the arrangement, the supply roller 33 and the toner holding member 31 are operated such that a uniform toner layer is formed on the toner holding member 31 by means of the regulating blade 32 and transported. Under these conditions, a voltage is applied to the rear electrode 38 and a voltage corresponding to the image signal is applied from the control power supply 37 to the control electrode 36 in synchronization with the movement of the image receiving means 40 while it is moved. Consequently, the toner on the toner holding member 31 passes through the toner passage hole 35 in response to the image signal to jump onto the image receiving means 40 and form a required image thereon.
To form a fine image at, e.g., 600 dpi (a density of 600 dots per inch) on the entire surface of the image receiving means 40, a plurality of toner passage holes 35 should be formed in the printhead 34. Even if the toner passage holes 35 are arranged in a row, a fine image as mentioned above cannot be formed so that the toner passage holes 35 and the control electrodes 36 are arranged in a plurality of rows (e.g., eight rows in an example shown in the drawing), as shown in FIG. 13. The toner passage holes 35 and the control electrodes 36 have, e.g., circular configurations. Connecting electrodes electrically connecting to each of the control electrodes 36 are provided on both sides of the toner holding member 31 to extend along the moving direction thereof for the prevention of mutual interference therebetween. Each of the connecting electrodes is connected to the lead of the control power supply 37 for outputting a control voltage, such as a driving IC.
Although the image receiving means 40 is composed of a recording sheet or the like such that an image is formed directly thereon in the exemplary structure shown in FIG. 12, the recording sheet or the like is disadvantageous in that the thickness thereof easily varies, the properties thereof easily change due to humidity, and it is prone to deformation during movement. In the case of color printing, it is difficult to synchronize the timings for image formations in different colors due to inconsistent transportation of the recording sheet so that image quality is likely to be lowered.
To prevent this, there are cases where an intermediate image holding belt is used preferably as the image receiving means 40 so that images formed on the image holding belt are transferred simultaneously onto a recording sheet or the like, as shown in the specification and drawings of, e.g., Japanese Patent Application No. HEI 10-100780.
Referring to FIG. 14, a description will be given to a structure using the image holding belt. In FIG. 14, 43 denotes an endless image holding belt as the image receiving means 40 which is composed of a resin film having conductive fillers dispersed therein and a resistivity of 1010 xcexa9cm. The image holding belt 43 is wound around a pair of rollers 44a and 44b. 45 denotes a pickup roller for feeding out recording sheets 46 on a one-by-one basis from a sheet feed tray 50. 47 denotes a timing roller for providing synchronism between the fed recording sheet 46 and the position of an image. 48 denotes a transfer roller for transferring a toner image formed on the image holding belt 43 onto the recording sheet 46. The transfer roller 48 is pressed toward the roller 44a with the image holding belt 43 interposed therebetween, while a transfer voltage is applied thereto. 49 denotes a fixing unit for fixing the toner image to the recording sheet 46 by heating and pressing the recording sheet 46 having the toner image transferred thereon.
However, the foregoing image forming apparatus has the problem of so-called clogging. As the apparatus is used for a longer period, toner particles gradually clog the toner passage holes (printhead openings) and hamper precise recording. As a method for preventing clogging, there has been known one which cleans the openings of the printhead by ejecting the toner from the toner holding member (toner supplying member) through the printhead openings.
Specifically, there have been proposed a method of applying an ultrasonic wave to the printhead, a method of applying a voltage to an electrode provided on the printhead, a method of forcibly pressing the toner supplying member onto the printhead, and the like.
However, if the toner ejected for cleaning in accordance with the foregoing conventional methods is large in amount, the toner deposited on the rear electrode reaches the printhead to enter the printhead openings. If the toner is further ejected under this condition, the deposited toner presses the toner in the printhead openings so that the toner solidifies in the printhead openings. This leads to the problem that clogging is caused by the toner ejected for cleaning.
Clogging resulting from the deposition of the ejected toner presents a serious problem not only in the cleaning step but also on other occasions. For example, clogging as described above occurs when a driving system for the rear electrode is out of order or when a driving roller for the intermediate transfer belt skids and the intermediate transfer belt rotates free.
If the toner is ejected in the clogging state, the toner solidifies in the openings, which resultingly damages the printhead.
There is still another problem that, if the toner is deposited on the opposing member during ejection, so-called in-apparatus contamination is likely to occur. On collision with the toner on the opposing member, the ejected toner is scattered to a periphery to contaminate the inside of the apparatus, thereby causing the in-apparatus contamination.
If a large mount of toner is deposited on the opposing member, the toner adheres to the printhead due to electrostatic repulsion between toner particles. As a result, an electric field is distorted by the charge of the toner adhered to the head so that the direction of the jumping toner is distorted.
The present invention has been achieved in view of the foregoing and it is therefore a primary object of the present invention to keep the developer deposited on the opposing member from contact with the printhead and prevent the clogging of the printhead openings.
To attain the object, the present invention provides a method for forming an image, the method comprising: an opposing member moving step of moving an opposing member disposed in opposing relation to a developer supplying member for supplying a developer with a printhead interposed therebetween; and an ejecting step of ejecting the developer from the developer supplying member toward the opposing member through openings of the printhead, the ejecting step being initiated after the opposing member moving step is initiated.
By allowing the opposing member to move, the arrangement prevents the developer from being deposited at a localized point on the opposing member, thereby keeping the developer on the opposing member from contact with the printhead and preventing the clogging of the openings. In addition, the probability of collision between developers on the opposing member is also reduced so that in-apparatus contamination due to the jumping of a toner is prevented.
The method can further comprise: a detecting step of detecting a moving state of the opposing member, wherein the initiation of the ejecting step is controlled based on a result of the detection obtained in the detecting step.
The method comprises: an opposing member moving step of moving an opposing member disposed in opposing relation to a developer supplying member for supplying a developer with a printhead interposed therebetween; and a cleaning step of ejecting the developer from the developer supplying member toward the opposing member through openings of the printhead and thereby cleaning the openings of the printhead, the cleaning step being initiated after the opposing member moving step is initiated.
By allowing the opposing member to move, the arrangement prevents the developer ejected during the cleaning of the openings from being deposited at a localized point on the opposing member, thereby keeping the developer on the opposing member from contact with the printhead and preventing the clogging of the openings. The foregoing effect is particularly significant in the cleaning step in which the developer is ejected in large amount per unit time.
A moving speed of the opposing member is adjusted to be higher in the cleaning step for the printhead than in an image forming step of forming a toner image.
The arrangement reduces the amount of deposited toner per unit area on the opposing member in the cleaning step for the printhead. As a result, the printhead can be cleaned by ejecting the toner in large amount.
In the image forming method, a plurality of printheads are arranged in a direction of movement of the opposing member and, of impact positions of developers ejected through each of the openings of the printheads and impacted on the opposing member in the cleaning step, at least one may be different from the others.
In the arrangement, the impact positions of the developers from the plurality of printheads are dispersed. This keeps the toners from contact with the printheads and prevents clogging due to local increases in the amounts of deposited toners.
The plurality of printheads correspond to the developers in a plurality of colors and, of the impact positions of the developers impacted on the opposing member in the cleaning step, the one in the palest color may be positioned closer to an image formation region on the opposing member than the impact positions of the developers in the other colors.
If the developers are ejected in accordance with, e.g., an ultrasonic method or a printhead contact method, the developers impacted on the opposing member are scattered to the peripheries of the impact positions. If the scattered developers enter the image formation region on the opposing member on which a recording member is disposed, the upper and lower end portions of the recording member is contaminated by the scattered developers. If the palest one of the developers in the plurality of colors, such as yellow, is deposited at the position closest to the image formation region, however, contamination becomes inconspicuous even if the developers enter the image formation region.
The impact position of the developer in the palest color can be positioned closer to a tip of the image formation region in a direction in which the opposing member is moved than the impact positions of the developers in the other colors.
In the arrangement, part of the scattered developers float in the space between the printheads and the opposing member to be impacted again on the opposing member so that the tip of the image formation region in the direction of movement is most likely to suffer contamination. By causing the palest one, such as yellow, of the developers to be impacted on the tip portion of the image formation region in the direction of movement, contamination on the tip portion of the image formation region in the direction of movement becomes inconspicuous.
An intensity of an electric field between the printhead and the opposing member can be adjusted to be lower in the cleaning step than in an image forming step of forming an image by ejecting the developer onto the opposing member.
Even if an electrostatic field formed between the printheads and the opposing member divides the developers into those of a positive polarity and those of a negative polarity, the arrangement prevents the developers of one of the positive and negative polarities from adhering to the surfaces of the printheads and allows the developers of each of the positive and negative polarities to reach the opposing member by using the electric field formed between the printhead and the opposing member. Accordingly, the efficiency with which the developers are reclaimed can be increased irrespective of the polarities of the developers.
The method can further comprise: a developer supplying step of supplying the developer from the developer supplying member to a vicinity of each of the openings of the printhead, wherein the cleaning step is initiated prior to the developer supplying step.
The arrangement prevents the ejection of the developer in large amount resulting from the cleaning step performed with a continuous supply of the developer and reduces the amount of the ejected developer by halting the supply of the developer in the cleaning step.
The method may further comprise: an electric field forming step of forming a specified electric field between a group of electrodes around the openings of the printhead and the developer supplying member, wherein the electric field forming step is initiated before the cleaning step is initiated.
By forming the electric field for suppressing the supply of the developer from the developer supplying member, the arrangement reduces the amount of the ejected developer.
The present invention also provides an apparatus for forming an image, the apparatus comprising: a developer supplying member for supplying a developer; an opposing member disposed in opposing relation to the developer supplying member; and a printhead disposed between the developer supplying member and the opposing member and having openings for allowing the developer on the developer supplying member to pass therethrough toward the opposing member based on an image signal from the outside, the opposing member initiates movement before the developer on the developer supplying member is ejected through the openings of the printhead.
By allowing the opposing member to move, the arrangement prevents the developer from being deposited at a localized point on the opposing member, thereby keeping the developer on the opposing member from contact with the printhead and preventing the clogging of the openings.
The apparatus may further comprise: detecting means for detecting the movement of the opposing member; and ejection initiation control means for controlling initiation of the ejection of the developer from the developer supplying means if the movement of the opposing member is detected by the detecting means.
There is also provided an apparatus for forming an image, the apparatus comprising: a developer supplying member for supplying a developer; an opposing member disposed in opposing relation to the developer supplying member; and a printhead disposed between the developer supplying member and the opposing member and having openings for allowing a developer on the developer supplying member to pass therethrough toward the opposing member based on an image signal from the outside, the developer on the developer supplying member being ejected through the openings of the printhead to clean the openings after the opposing member initiates movement.
By allowing the opposing member to move, the arrangement prevents the developer ejected during the cleaning of the openings from being deposited at a localized point on the opposing member, thereby keeping the developer on the opposing member from contact with the printhead and preventing the clogging of the openings. The foregoing effect is particularly significant in the cleaning state in which the developer is ejected in large amount per unit time.
A moving speed of the opposing member can be adjusted to be higher when the openings are cleaned by ejecting the developer on the developer supplying means through the openings of the printhead than when a toner image is formed.
The arrangement reduces the amount of deposited toner per unit area on the opposing member during the cleaning of the printhead. As a result, the printhead can be cleaned by ejecting the toner in large amount.
In the image forming apparatus, a plurality of printheads are arranged in a direction of movement of the opposing member and, of impact positions of developers ejected through each of the openings of the printheads and impacted on the opposing member during a cleaning period for each of the printheads, at least one may be different from the others.
In the arrangement, the impact positions of the developers from the plurality of printheads are dispersed. This keeps the toners from contact with the printheads and prevents clogging due to local increases in the amounts of deposited toners.
The plurality of printheads correspond to the developers in a plurality of colors and, of the impact positions of the developers impacted on the opposing member during the cleaning period for each of the printheads, the one of the palest color can be positioned closer to an image formation region on the opposing member than the impact positions of the developers in the other colors.
If the developers are ejected in accordance with, e.g., an ultrasonic method or a printhead contact method, the developers impacted on the opposing member are scattered to the peripheries of the impact positions. If the scattered developers enter the image formation region on the opposing member on which a recording member is disposed, the upper and lower end portions of the recording member is contaminated by the scattered developers. By depositing the palest one of the developers in the plurality of colors, such as yellow, at the position closest to the image formation region, as described above, the present invention can render contamination inconspicuous even if the developers enter the image formation region.
In the image forming apparatus, the impact position of the developer in the palest color may be positioned closer to a tip of the image formation region on the opposing member in the direction of movement of the opposing member than the impact positions of the developers in the other colors.
In the arrangement, part of the scattered developers float in the space between the printheads and the opposing member to be impacted again on the opposing member so that the tip of the image formation region in the direction of movement of the opposing member is most likely to suffer contamination. By causing the palest one of the developers, such as yellow, to be impacted on the tip portion of the image formation region, contamination on the tip portion of the image formation region becomes inconspicuous.
In the image forming apparatus, an intensity of an electric field between the printhead and the opposing member can be adjusted to be lower during the cleaning period than during image formation during which an image is formed by ejecting the developer onto the opposing member.
Even if an electrostatic field formed between the printheads and the opposing member divides the developers into those of a positive polarity and those of a negative polarity, the arrangement prevents the developers of one of the positive and negative polarities from adhering to the surfaces of the printheads and allows the developers of each of the positive and negative polarities to reach the opposing member by using the electric field formed between the printhead and the opposing member. Accordingly, the efficiency with which the developers are reclaimed can be increased irrespective of the polarities of the developers.
The cleaning of the openings of the printhead is initiated before the developer is supplied from the developer supplying member to a vicinity of each of the openings of the printhead.
The arrangement prevents the ejection of the developer in large amount resulting from the cleaning step performed with a continuous supply of the developer and reduces the amount of the ejected developer by halting the supply of the developer in the cleaning step.
A group of electrodes are provided around the openings of the printhead and a specified electric field is formed between the group of electrodes and the developer supplying member.
By forming the electric field for suppressing the supply of the developer from the developer supplying member, the arrangement reduces the amount of the ejected developer.
The developer that can be used in the present invention is not limited to a dry toner. Other developers including a liquid ink having a pigment or dye dispersed in a fluid dispersion and a wet toner having toner particles dispersed in a fluid dispersion may also be used.
If the liquid ink is used, the developer ejected toward the opposing member such as an intermediate transfer belt or a rear electrode is not deposited as a toner so that the clogging of the openings by the developer deposited on the opposing member does not occur. However, if the liquid ink is impacted in large amount on the same impact position, the opposing member swells or the mechanical strength thereof is reduced significantly due to the penetration of the liquid ink into the opposing member. This causes the problems of a shorter lifetime of the opposing member, degraded registration of a color image due to a change in the size of the opposing member, and a damaged peripheral member such as a damaged printhead due to the deformation of the opposing member. However, the present invention can reduce the foregoing problems since the impact positions of the liquid ink are dispersed. If the liquid ink is impacted on a hard opposing member, the phenomenon of the ink scattered to the periphery of the opposing member may also occur, similarly to the dry toner. Hence, the present invention is effective in either case in solving the problems caused by the phenomenon.
If the wet toner is used, the fluid dispersion reaches the opposing member together with the toner particles. This causes two problems, of which one is associated with the deposition of the toner on the opposing member also observed when the dry toner is used and the other is associated with the penetration of the fluid dispersion into the opposing member also observed when the liquid ink is used. By practicing the present invention, therefore, these problems can be solved.